Structural materials obtained by shaping sheet-like fiber-reinforced plastics through stamping molding are widely used in various fields such as aircraft members, automobile members, wind turbine members for wind power generation, and sports goods. The fiber-reinforced plastic is formed, for example, by laminating and integrating a plurality of prepreg base materials in which reinforcing fibers are impregnated with a thermoplastic resin.
Examples of the prepreg base material may include those obtained by impregnating those obtained by pulling and aligning continuous reinforcing fibers having a long fiber length in one direction with a thermoplastic resin and then forming into a sheet shape. A structural material exhibiting excellent mechanical properties can be produced from a fiber-reinforced plastic formed from a prepreg base material using such a continuous long reinforcing fiber. However, it is difficult to shape the fiber-reinforced plastic into a complicated shape such as a three-dimensional shape since it is formed from a continuous reinforcing fiber and the fluidity at the time of shaping is thus low. Hence, the structural material to be produced is limited mainly to those close to a planar shape in the case of using the fiber-reinforced plastic.
As a method for enhancing the fluidity at the time of shaping, for example, a method (I) in which a plurality of prepreg base materials obtained by impregnating reinforcing fibers pulled and aligned in one direction with a thermoplastic resin and forming cuts intersecting the fiber axis are laminated and integrated into a fiber-reinforced plastic are proposed (Patent Literatures 1 to 5). The fiber-reinforced plastic obtained by the method (I) exhibits favorable fluidity at the time of shaping since cuts are formed on the prepreg base material and the reinforcing fibers are thus cut. In addition, it is possible to obtain a fiber-reinforced plastic exhibiting favorable isotropy of mechanical properties and small variations by laminating a plurality of prepreg base materials so that the fiber axis directions of the reinforcing fibers are not biased in a specific direction, for example, fiber axis directions are deviated by 45° in a planar view. The anisotropy can also be controlled by laminating the prepreg base materials so that the fiber axe directions are aligned in an arbitrary direction.
However, in the method (I), problems such as deteriorated handleability arises in some cases as swelling and warpage occur to increase the wound diameter when a long fiber-reinforced plastic is produced and wound up.
In addition, the method (I) is not yet satisfactory from the viewpoint of ease of production and cost. Particularly in the industrial production of fiber-reinforced plastics and structural materials (molded articles), it is important to be able to produce them more easily and inexpensively. In addition, it is important to further enhance the fluidity at the time of shaping of the fiber-reinforced plastic in order to obtain a structural material having a more complicated shape.
In addition, as a method for enhancing the fluidity at the time of shaping, there is a method in which discontinuous reinforcing fibers are randomly disposed. For example, a method (II) in which a plurality of prepreg pieces cut at a certain length from a tape-like prepreg base material having a narrow width are dispersed on a flat surface and integrated into a sheet-like fiber-reinforced plastic through press molding is disclosed (Patent Literature 6).
However, in the method (II), it is extremely difficult to uniformly disperse the prepreg pieces so that the fiber axis directions of the reinforcing fibers are directed in completely random directions since the prepreg pieces are dispersed by flying the prepreg pieces in the air or diffusing the prepreg pieces in a liquid fluid and then depositing them. Hence, a fiber-reinforced plastic having different mechanical properties such as strength depending on the location and direction is obtained even in the same sheet. It is often demanded to a structural material that the variations in mechanical properties such as strength are small and the mechanical properties are isotropic or the anisotropy thereof is controlled. However, by the method (II), it is difficult to obtain a fiber-reinforced plastic in which isotropy of mechanical properties is favorable or anisotropy thereof is controlled and further the variations in mechanical properties are small.
Moreover, a fiber-reinforced plastic is also demanded to exhibit favorable heat resistance. Generally, the heat resistance of fiber-reinforced plastic is greatly affected by the heat resistance of the matrix resin to be used in the fiber-reinforced plastic. Usually, the mechanical properties of a resin simple substance tend to decrease at temperatures equal to or higher than the glass transition temperature of the resin. The mechanical properties of the fiber-reinforced plastic also tend to decrease at temperatures equal to or higher than the glass transition temperature of the matrix resin in the same manner. In order to minimize this decrease in mechanical properties, it is required to uniformly disperse the reinforcing fibers in the matrix resin in the fiber-reinforced plastic. However, in the method (II), only the molten matrix resin flows into the gaps between the deposited prepreg pieces in the step of heating and integrating the deposited prepreg pieces. Hence, a resin-rich portion is locally formed in the fiber-reinforced plastic obtained. The fiber-reinforced plastic obtained by the method (II) has a problem of inferior heat resistance due to the influence of this resin-rich portion.
In addition, the fiber-reinforced plastic obtained by the method (I) has a problem that mechanical properties decrease as the cut portion serves as a starting point of fracture in a case in which a stress is generated in the direction along the shape of cut. In addition, there is a problem that the heat resistance is inferior at temperatures equal to or higher than the glass transition temperature of the matrix resin in the same manner as in the method (II) disclosed in Patent Literature 6 since only the resin is substantially present at this cut portion.
In addition, in the method (I), it is required to separately produce band-like prepreg base materials in which the fiber axis directions of the reinforcing fibers are set to different directions (for example, 0°, 45°, 90°, and −45° with respect to the machine direction) in a planar view and to laminate them in the case of continuously producing a band-like fiber-reinforced plastic exhibiting favorable isotropy of mechanical properties.
Hence, the production process is complicated, the control is difficult, and the cost increases. In addition, it is required to laminate the prepreg base materials while rotating each prepreg base material at a predetermined rotation angle (0°, 45°, 90°, −45°, or the like) at all times so that the fiber axis directions of the reinforcing fibers are not biased in a planar view in the case of producing a sheet-like fiber-reinforced plastic as well. Hence, the laminating operation is complicated, the control is difficult, and the cost increases in this case as well.
Patent Literature 7 discloses a method (III) for producing a fiber-reinforced plastic by dispersing reinforcing fibers through papermaking. The fiber-reinforced plastic obtained by the method (III) exhibits excellent isotropy of mechanical properties, small variations, and favorable heat resistance since the reinforcing fibers are almost uniformly dispersed in the fiber-reinforced plastic.
However, the fiber-reinforced plastic obtained by the method (III) exhibits inferior fluidity at the time of shaping since the reinforcing fibers are three-dimensionally entangled in the fiber-reinforced plastic. In addition, the production process is also extremely complicated and the fiber-reinforced plastic is inferior in cost. Moreover, it is required to conduct papermaking of reinforcing fibers in a denser state in the case of attempting to produce a fiber-reinforced plastic having a high content of reinforcing fibers by the method (III). However, particularly the reinforcing fibers oriented in the thickness direction (impregnated direction) are responsible for the stress of the pressing force at the time of impregnation among the three-dimensionally entangled reinforcing fibers when it is attempted to impregnate the reinforcing fibers subjected to such papermaking in a high density with a matrix resin, and thus the pressure is not transmitted to the resin and the impregnation is extremely difficult. In addition, three-dimensional entanglement is strengthened in a case in which the fiber length of the reinforcing fibers is long as well, and the impregnation is thus difficult in the same manner.
In addition, the conventional fiber-reinforced plastics in which discontinuous fibers are randomly disposed do not necessarily exhibit sufficient mechanical characteristics and an improvement in modulus of elasticity is desired.
In the case of producing a structural material by shaping a fiber-reinforced plastic into a complicated shape such as a three-dimensional shape, a relatively short reinforcing fiber having a fiber length of 100 mm or less is generally used in order to ensure the fluidity at the time of shaping. However, the mechanical properties of the structural material after shaping are likely to decrease when the fiber length of the reinforcing fibers is shortened.
As a fiber-reinforced plastic which can provide a structural material exhibiting high mechanical properties while a using discontinuous reinforcing fiber, the following ones are proposed.
(1) A fiber-reinforced plastic which is constituted by a reinforcing fiber having a fiber length of from 5 to 100 mm and a thermoplastic resin and in which the average value of tan δ′ in a range of the melting point of the thermoplastic resin±25° C. satisfies 0.01≤tan δ′ (average value)≤0.2 (Patent Literature 8). Tan δ′ is defined by the following two equations.Tan δ=G″/G′Tan δ′=Vf×tan δ/(100−Vf)(where Vf is the volume fraction (%) of the reinforcing fiber in the fiber-reinforced plastic, G′ is the storage modulus of elasticity (Pa) of the fiber-reinforced plastic, and G″ is the loss modulus of elasticity (Pa) of the fiber-reinforced plastic).
The fiber-reinforced plastic of (1) exhibits favorable mechanical properties as tan δ′ (average value) is controlled in a specific range.
However, in the fiber-reinforced plastic of (1), tan δ′ hardly changes even in a state in which the nylon resin (melting point: 225° C.) of a thermoplastic resin is heated to the melting point as described in Examples 1 and 2. The fact that tan δ′ hardly changes indicates that the value of tan δ itself hardly changes since Vf/(100−Vf) is constant, but the reinforcing fibers and the thermoplastic resin do not exhibit sufficiently high fluidity when being heated at the time of shaping and the reinforcing fibers are cut and shortened at the time of shaping when the value of tan δ is small even in a temperature range to be equal to or higher than the melting point and the property as an elastic body is great, and there is thus a possibility that the mechanical properties of the structural material after shaping are insufficient.